The pore structure of the 40wt% Ni/kieselguhr catalyst prepared by precipitation of nickel component from a 0.17M nickel nitrate solution at 90℃ changes when the precipitant, 1M Na₂CO₃ solution, is added to the nickel nitrate solution over different periods. After precipitation for 22 hours, the catalyst experiences narrowing of the pore mouth which leads to appearance of new small pores. However, when the catalyst is prepared by precipitation for 2 hours, the small pores present in the kieselguhr support are blocked by the precipitate particles and only the large pores present in the kieselguhr support are blocked by the precipitate particles and only the large pores are remained in the catalyst. The catalyst containing relatively small pores exhibits low reaction rates and changes in the product selectivity in soybean-oil hydrogenation, due to the diffusional resistance of the reactants in the catalyst pores. The effect of the diffusional resistance is not observed with CO hydrogenation, benzene hydrogenation and n-hexane hydrogenolysis, where molecular sizes of the reactants are significantly smaller than the catalyst pores.